The present invention concerns a biaxially-oriented film made up of a base layer and at least one top layer which contains at least one polymer made up of at least one aliphatic hydryoxycarboxylic acid. The invention furthermore concerns a procedure for manufacturing said film, as well as its use.
Films made of synthetic thermoplastics are introduced on a large scale for packaging foodstuffs and other packaged goods. It is essential for these uses that the films feature good antistatic and dynamic friction properties.
Newer developments in the area of packaging concern films made of biodegradable polyesters, like e.g. polylactic acid (PLA). These kinds of films count as particularly environmentally-friendly, as they are based on renewable raw materials and can be disposed of by composting. These materials are, however, fundamentally different to the olefinic polymers like polyethylene or polypropylene, which are introduced for packaging films on a large scale. The transfer of technical teachings about polyester films also often does not succeed, as the same or similar measures often do not achieve the same effect with PLA films.
A comparably good spectrum of usage properties is nevertheless necessary for economic success, in order that this film can be handled with the conventional packaging technologies. Here, the static inhibition of the film in particular, in combination with its dynamic friction properties, plays a central role. Numerous investigations have shown that the migrating additives conventional in the boPP area, which are introduced to improve these properties, do not stand up to testing in the PLA films. Because of the polar character of the PLA polymers, these migrating additives in the polymer matrix show quite different migratory behaviour. Often, these substances arrive not at all, or only in small quantities, on the surface of the PLA film, so that they remain ultimately without effect.
Moreover, in the prior art the introduction of inorganic or organic anti-blocking particles into the top layers of a film as spacers between individual film layers is known. The anti-blocking particles can improve the winding behaviour of a film roll by reducing the contact surface between the film layers, and in addition beneficially influence the friction properties between film sheet and guiding rails on a packaging machine. In general, however, anti-blocking agents do not contribute towards improved antistatic properties of the film.
In the German application number 101 21 153.8, a PLA film is described which contains particularly high quantities of glycerine monostearate in order to improve the static inhibition. However, these films are still in need of improvement.
The addition of high quantities of particles to improve friction is a known solution with known polyester films made of polyethylene terephthalate, but this modification also leads to no improved static inhibition in PLA films with the conventional particles.
Moreover, various effective combinations of migrating additives and particle-forming anti-blocking agents are known from the boPP technologies, which are introduced in order to improve friction and static inhibition. Within the scope of the present invention it was found, however, that these combinations in PLA film, which do not improve static inhibition, even partly have a negative effect, either as particles or additives. It is supposed that the particle-forming additives in the PLA matrix influence the migrating behaviour of the additives, for example some types of particles seem to absorb the migrating additives so that they remain near-ineffective.